The present invention generally relates to audio amplifiers and more particularly to Class D audio amplifiers. The invention is still more particularly directed to the problem of noise in Class D amplifiers. In audio reproduction and reinforcement systems, noise is defined as any sound produced by the system which is not an intentional part of the audio program. Noise is objectionable and particularly noticeable when the audio channel is idle.
Class D audio power amplifiers (switching amplifiers) offer a substantial improvement in efficiency over linear Class AB power amplifiers. Class D amplifiers, however, have drawbacks, which includes higher noise. Class D amplifiers achieve high efficiency only when the output power transistors can be operated in a low loss mode, meaning they are predominantly fully on (saturated state), or fully off, operating like an ideal switch. In practice power loss occurs not only from the resistive loses while the transistor is on but also during the transition state where the transistor is changing from an on or off state. During the transition time there is a high product of voltage and current which produces dissipation in the transistor. Since electrical and physical limitations prevent fast transition of on and off states, the switching (clocking) frequency of Class D amplifiers cannot be too high, particularly at higher voltages and currents needed for higher wattages. In practice, most Class D amplifiers of wattages greater than 50 watts switch at frequencies below 500 kHz.
Class D amplifiers often use modern analog to digital converters (ADCs). Modern high quality audio ADCs utilize sigma-delta conversion with a high degree of oversampling, filtering, and decimating to achieve low quantization noise on their outputs. The pulse width modulator (PWM), which creates the pulses to control the Class D power output transistors, cannot operate at as high degree of oversampling as the ADC, due to the switching speed limitation of the output transistors. This poses a challenge for designing low noise digital PWMs for Class D amplifiers since techniques of oversampling, interpolating, filtering, for noise reduction are more limited compared to ADCs and DACs. Therefore, noise introduced by the digital PWM alone can be significant.
Typically, Class D amplifiers with digital PWMs will achieve 90-110 dB dynamic range, whereas linear Class AB amplifiers can more readily produce 110-120 dB dynamic range. Highest quality digital PWMs for Class D amplifiers offer around 110 dB, but generally that is the upper limit. When low noise audio amplifiers are needed, Class D amplifiers are therefore not favored.